How to optimize LINQ-to-NHibernate Lazy Loading

Question

Question: Given an NHibernate entity Parent, is there any way to get a LINQ query on a Children collection to execute on the database side rather than lazy-loading all the children and then performing the query?

Scenario:

Parent object has IList collection (Children)

var parent = _parentRepository.Get(parentId); //loads parent

..do stuff

//this causes all Child objects to be loaded into memory
//and then finds the subset of boy objects (not great performance)
var boys = parent.Children.Where(t => t.Sex == 1);

If I try explicitly passing an Expression<Func<Child, bool>>, I get an error that it's expecting a type Func<Child, bool>.

Is there any way to get more efficient lazy loading w/NHibernate?

Thanks!

Gert Arnold · Accepted Answer · 2011-08-20 15:06:41Z

2

Recommended practice in NHibernate is to keep a session short-lived ans thus prevent lazy loading. You can improve the efficiency of your query by applying "join fetch" (refer to the NHibernate documentation), which, by the way, will also read all child objects, but in one shot and not in the infamous 1 + N anti pattern.

Children is not an IQueryable so you can't use an expression. Linq to NHibernate would allow you to query a Session with linq statements that get translated into sql. Then you could query the Children collection with expressions as predicates.

answered Aug 20, 2011 at 15:06

Gert Arnold

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1 Comment

sydneyos Over a year ago

Yeah, that's what I was afraid of - was hoping someone more clever than I had designed a more efficient approach.

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